Size Effect Of Gas Sensitive SnO₂ Nanocrystalline And Key Parameters In The Gas-sensing Mechanism:A First Principle Study

The development of nanoscience has accelerated the process of gas sensor entering the era of quantum dot.In view of the excellent performances such as high sensitivity and fast response of quantum dot gas-sensitive materials,the paper uses the first-principles method to discuss the size effect of gas sensitive SnO2 nanocrystalline from the micro-electronic level,and then conduct research on the key parameters of the gas-sensing mechanism.The relation-ship between the first-principles calculation results and the experimental parameters is estab-lished through the exponent of the power law of resistance,and a new method for verifying the simulation results by experiments is established to solve the problem that the simulation results are difficult to measure.In this work,the first principle calculation based on the density function theory,the den-sity of states of SnO2 with complete and defective SnO2 crystallite sizes ranging from 0.474 to 1.593 nm are calculated.And the formation of oxygen vacancies and adsorption of oxygen species in the SnO2 semiconductor are calculated,exploring the size effect on oxygen vacan-cies and the adsorbates of super-oxide ions.The results show that the carrier concentration and electrical conductivity can be improved by increasing the SnO2 crystallite size.The for-mation energies of oxygen defects in complete and defective SnO2 super cells are of positive correlation with crystallite size,revealing vacancies are most likely to appear at the surface bridge position.The adsorption energies of O-and O2-species on defective SnO2 super cells increase with crystallite size.The Mulliken population distribution is used to analyze the elec-tron transfer in the SnO2 super cells,which illustrates that the O 2p orbit captures the elec-trons from the orbits of Sn 5s and 5p as well as O 2s.With the assistance of connecting Sn atoms,the adsorbates of O-and O2-are able to capture electrons from the inner region of crystallites,resulting in an expansion of depletion layer.On the surface of SnO2 crystallites,there is mutual conversion between adsorbed oxygen O-and O2-.And based on this,an adsorbed oxygen conversion model is established.The cal-culation of the adsorption energy of the conversion model shows that when the adsorbed ox-ygen state is O2-:O-=12:6,the absolute value of the conversion energy AE is the smallest,which is 0.008 eV,indicating that this state has the highest probability of occurrence.The type of adsorbed oxygen determines the power law index m of resistivity.At 350?,the propor-tion of O2-and O-is 66.7%and 33.3%,which happens to be in highest probability state.The theoretical m is 5/6.At 25?,the proportions of O2-and O-in the atmosphere is 91%and 9%,and the theoretical m is 0.955.And in this paper,the SnO2 quantum dot sensors are prepared.At 25?,the SnO2 quantum dot sensors are placed in a reducing atmosphere H2 and C4H10,and the resistance and response of the sensor connected to the circuit are measured,and m are 0.914 and 0.852,respectively.Through comparison,it is found that the theoretical m is very close to the experimental m.Therefore,in this paper,the relationship between the first-principles calculation results and the experimental parameters is established through the exponent of the power law of resistance.In this work,by the Mulliken population distribution revealing the expansion of the de-pletion layer,from the perspective of the model,the Debye length(LD)is calculated.The three-dimensional SnO2 super cell is reduced into the two dimensions lattice sequence.Based on the transfer charge between atoms,in the SnO2 two dimensions lattice sequence to find the smallest charge transfer between the acceptor and atomic combination,the shortest path of this combination is LD.It is calculated that the LD of 1.421nm SnO2 is 4.801 A.And based on the parameters of Debye length model,the Schottky approximation method is used to calcu-late LD.The theoretical LD is 5.27A.The results of the two calculation methods are in good agreement,and based on the model,a new method for estimating Debye length is established.